1. Field of the Invention
The invention is directed to drill string compensators, and in particular to portable drill string compensators for use in connection with off-shore drilling operations such as off-shore drilling vessels to permit vertical movement of the drill string in relation to ocean heave.
2. Description of Related Art
Drill string compensators are employed to compensate for vessel motion induced by wave action and heave. Drill string compensators are also utilized to maintain a variable tension to the drill string alleviating the potential for compression and in turn buckling or failure.
Historically, conventional drill string compensators have consisted of both single and dual cylinder assemblies with a chain fixed at one end of the cylinder and a movable chain sheave attached to the rod end of the cylinder as disclosed in U.S. Pat. No. 3,804,183. The assembly is then mounted in a position on the vessel to allow convenient routing of chain which is connected to a point at the fixed end and strung over the movable sheaves. In turn, the chain is routed via sheaves and connected to the drill string compensator via a support consisting of a hook which is connected to the end termination of the chain assembly.
The cylinders and the chain assemblies are disposed on a derrick disposed above the drill string. Also disposed on the derrick, or on deck space located remotely from the derrick, but in close proximity, is a hydro/pneumatic system consisting of high pressure air vessels. Pressure from the air pressure vessels (“APVs”) forces the rod and in turn the rod end sheave to stroke out thereby tensioning the chain and in turn the drill string.
One drill string compensator typically used on a rig and is set to support a portion of th weight of the drill string. The remaining portion of the drill string weight provides the force necessary for penetration as the drill string is spun.
Normal operation of these conventional type drill string compensator systems have required high maintenance due to the constant motion producing wear and degradation of the chain members. In addition, available space for installation and, the structure necessary to support the units including weight and loads imposed, particularly in deep water applications where the tension necessary requires additional large drill strings poses difficult problems for system configurations for both new vessel designs and upgrading existing vessel designs.
Additionally, as disclosed in U.S. Pat. No. 3,793,835, in prior drill string compensators, a bank of remotely located, either along the derrick or on the deck of the vessel, hydraulic fluid accumulators and APVs are required. These hydraulic fluid accumulators and APVs require large amounts of deck space with heavy piping and large diameter hoses to provide the operating pressure to the drill string compensator. These hoses combined with the control lines create bulky, heavy hose bundles, thereby requiring additional space and adding additional weight to the drilling vessel. Therefore, the portability of these drill string compensators is severely limited.
Accordingly, prior to the development of the present invention, there has been no drill string compensators or methods of compensating a drill string, which: provide portability to the entire drill sting compensator system, including APVs and hydraulic fluid accumulators; reduce the weight of equipment necessary to operate the drill string compensators; reduce the amount of deck space required for the drill string compensators; provide a self-contained and compact drill string compensator; and are operable without the use of a separate derrick. Therefore, the art has sought a drill string compensator and a method of compensating a drill string, which: provide portability to the entire drill sting compensator system, including APVs and hydraulic fluid accumulators; reduce the weight of equipment necessary to operate the drill string compensators; reduce the amount of deck space required for the drill string compensators; provide a self-contained and compact drill string compensator; and are operable without the use of a separate derrick.